Could also be that the installer of the game simply wants to install DirectX anytime and simply aborts after finding a newer version.

Half right. Usually Steam has a special run script that it executes every time a game is launched. Part of this run script is installing any missing dependencies when the game is first launched. Some of these dependencies can include the appropriate DX Runtimes.
This is most likely what happened, that Steam ran the installer for the DX Runtimes needed for that game which just happened to use the same runtime version(s) as D3D9Client.

Basically all post win2000-systems were delivered with DX9+. Might still be an issue that the client requires a certain service pack, but with Win10? That's got DX12 bundled with it. What I could imagine is that your GPU drivers were severly out of date or badly installed, and that steam fixed that. I know Steam has the capability to do that, but usually it doesn't do it without asking... ?

Actually, the DX9 end-user runtimes aren't shipped as part of Windows now, and must be downloaded separately (https://www.microsoft.com/en-us/down...ils.aspx?id=35) for any program <DX10. Edit: Note: This includes Orbiter. I'm not sure which version of Windows ceased to ship with the runtimes, but I remember having to install them into 7.

Be great if the Dx11 client was continued for Orbiter 2016, give some future proofing to the software and better efficiency in handling system resources. I can see Dx9 falling into the same path of Dx7 quite soon with Windows 10 and later versions